Build and send a book only when it changed

A key of the chapters, rules and metadata of a story tells if its book
changed. A check without a change no longer builds the book again, also
for a book that waits for an upload. A held book goes out without a new
build. The Rebuild button still builds and sends in each case.

tests/test_worker_flows.py

@@ -25,6 +25,8 @@ def state(monkeypatch, tmp_path):
     monkeypatch.setattr(o, "job_queue", queue)
     monkeypatch.setattr(config, "CWA_URL", "")
     monkeypatch.setattr(config, "SITE_DELAY", 7)
+    monkeypatch.setattr(config, "TMP_DIR", tmp_path / "tmp")
+    config.TMP_DIR.mkdir()
     return store, queue
 
 
@@ -151,7 +153,8 @@ def site(state, monkeypatch):
         calibre={"title": "Title"},
     )
     fetch = Mock(side_effect=lambda a, url, i: f"<p>fresh {url}</p>")
-    build, deliver = Mock(), Mock(return_value="Sent")
+    build = Mock(side_effect=lambda story, overrides, path: path.write_bytes(b"epub"))
+    deliver = Mock(return_value="Sent")
     monkeypatch.setattr(fff, "get_adapter", lambda url: adapter)
     monkeypatch.setattr(fff, "fetch_story", lambda a: snap)
     monkeypatch.setattr(fff, "fetch_chapter", fetch)